1. Field of the Invention
This invention relates to a coated plastic film, and more particularly, to a coated plastic film having good printability, slip properties, oxygen and moisture barrier properties and transparency.
2. Description of the Prior Art
It is well known to coat a regenerated cellulose, biaxially oriented polypropylene, polyethylene terephtharate, nylon 6 or like film with a composition comprising mainly a resin such as a vinylidene or vinyl chloride copolymer to provide it with oxygen and moisture barrier properties. The coated film is suitable as packaging film material, and used after its coated surface is printed, or otherwise treated. The printing and other treatment of the coated film essentially requires its high slip properties. Therefore, it is usual practice to mix a fine powder in the coating solution.
The fine powder of kaolin, silica, bentonite, calcium carbonate, or the like is usually mixed in the coating solution. These inorganic fine powders can be used, whether the coating solution is a solution prepared with an organic solvent, or an aqueous dispersion. In addition to these inorganic fine powders, it has been proposed to use a fine powder of polystyrene as shown in Japanese Patent Publication No. 8452/73, a fine powder of polyethylene as in Japanese Patent Application Laid-Open No. 2942/71, and a fine powder of polystyrene or polyvinyl chloride as in Japanese Patent Application Laid-Open No. 99638/74. These powders of synthetic resins can, however, be used only when the coating solution is an aqueous dispersion. If the coating solution is a solution prepared by an organic solvent, the addition of any such synthetic resin powder is hardly expected to improve the slip properties of the coated film, since the powder is considerably swollen or dissolved by an organic solvent.
The printing of a packaging film is in most cases done by photogravure. Photogravure is the most suitable process for reproducing a special tone of a color photograph, depending on the depth and area of the cells on a gravure plate. If the cells have a depth of 5 to 15 microns, pinholes (unprinted portions) are likely to appear in the printed surface layer on the coated plastic film on which printing has been made, if the coating liquid contains a fine powder of an inorganic substance such as kaolin, silica, bentonite or calcium carbonate.
The inventors of this invention have looked into the cause of those pinholes, and found that they are due to surface conditions of the coated film to be printed i.e., projections existing on the coated surface. They have found that those projections are formed if the coating solution contains a fine powder of an inorganic substance such as kaolin, silica, bentonite or calcium carbonate. Accordingly, they have conducted printing tests by selecting fine powders having a relatively small average particle diameter, but encountered a limitation to the reduction in the particle diameter of any such powder, since the particle diameter must be greater than the thickness of the coating on the film in order to improve its slip and anti-blocking properties. Moreover, those inorganic fine powders contain particles having a diameter which is by far greater than the average diameter of the powder as a result of agglomeration. These particles form large projections on the coated surface, and give rise to pinholes when printing is made thereon. Therefore, it has been difficult to obtain a coated film having good printability leaving no pinhole in the printed surface layer thereof, while maintaining its excellent slip and anti-blocking properties.